This invention relates to an elastic fluid axial flow turbine or compressor and, more particularly, to the blading for such a turbine or compressor.
A variety of blade structures for turbines and compressors are known and a variety of techniques for minimizing vibratory stresses in the blade structure have been developed. For example, attention is directed to U.S. Pat. Nos. .[.3,279,751.]..Iadd.3,279,571.Iaddend.; 3,588,278; 3,417,964; 3,606,578; and 3,702,221, all of which are incorporated herein by this reference. Techniques for eliminating fatigue failures in turbine blading are also described by Ortolano, et al., "Long Arc Shrouding-Reliability Improvement for Untuned Steam Turbine Blading," ASME Publication 77-JPGC-Pwr-12, September, 1977, which also is incorporated herein by this reference.
An elastic fluid axial flow turbine or compressor comprises a rotor having a peripheral groove and an annular row of blades having root portions disposed in the groove. Small groups of blades have commonly been tied together to reduce vibratory stresses. In this technique, from about 3 to 12 blades are secured to each other by means such as a shroud, lashing wires, or both. However, a disadvantage of this method is that low orders of harmonic excitation are insufficiently attenuated to prevent fatigue failure, particularly when wet and corrosive steam is used.
To minimize this problem, the rotor blades have been connected together in long arc groups of more than 12 blades per group as described in U.S. Pat. No. 3,588,278. In this structure the number of blade groups is equal to .[.an integer multiple of.]. the resonant frequency of the blades divided by .Iadd.an integer multiple of .Iaddend.the rotor running speed. The blades can be connected to each other by a shroud structure or a lashing structure.
Although long arc shrouding has been successful in eliminating failure of untuned blading, failure of tuned blading can still occur because of gaps between the groups of blades. It is necessary to leave gaps because of thermal and centrifugal stresses that develop in use. Because of the gaps, axial-torsional modes of vibration can develop high vibration amplitudes which can lead to failure of blading.
Some solutions to this problem that have been considered include straps covering the gaps, pins bridging the gap between adjacent blades, and welds across the gaps. One such solution is described in U.S. Pat. No. 3,702,221.
These concepts are not totally satisfactory, because they have the disadvantage of not being applicable to existing blading without major modification. With some rotor structures, it is not possible to retrofit existing blading economically. Other problems with these techniques are limited resistance to moisture impingment erosion, an important factor with long low pressure blading.
In view of this, there is a need for a rotor structure that has the advantage of connecting the blades in long arc groups to suppress tangential and axial modes of vibration, and that also suppresses axial-torsional modes of vibration, improves moisture impingement erosion resistance, and can be retrofitted economically to existing rotor structures.